1. Field of the Invention
The present invention relates to a switching-regulator type DC-DC converter which converts direct-current voltage, and especially to a switching-mode power supply device which controls an output using a PFM (pulse-frequency modulation) technique.
2. Description of Related Art
A switching-regulator type DC-DC converter is an example of a circuit which outputs direct-current voltage having different electrical potentials by converting direct-current input voltage. As such a DC-DC converter, there is one that is provided with a driving switching element which applies a direct-current voltage supplied from a direct-current power supply such as a battery to an inductor (coil), passing a current thereto, and charges energy in the coil, a rectifying element which rectifies a current in the coil during an energy release period when the driving switching element is turned off, and a control circuit which performs on-off control of the driving switching element.
Conventionally, in the above-mentioned switching-regulator type DC-DC converter, a voltage proportional to an output voltage is fed back to a comparator for PFM (pulse-frequency modulation) control or to a comparator for PWM (pulse-width modulation) control, and therefore a frequency or pulse width is controlled to extend an ON period of the driving switching element when an output voltage decreases and a frequency or pulse width is controlled to shorten an ON period of the driving switching element when an output voltage increases.
FIG. 7 shows an outline configuration of a DC-DC converter using a PFM control technique. In the PFM control, an output voltage and a predetermined reference voltage Vref are compared to each other by a comparator CMP. When the output voltage is lower than the reference voltage Vref, an output of the comparator CMP is changed and a flip-flop FF is set, the output Q is changed to a high level turning on a driving switching element M1 on the high side and turning off a switching element M2 on the low side. Also, a timer TMR1 which defines an ON period based on the output Q of FF is activated, an output of the timer changes after a given period of time, and the flip-flop FF is reset, turning M1 off and turning M2 on. By having such structure, a pulse width, in other words, an ON period of the driving switching element is fixed, and control is performed so that a switching frequency is lowered when a load is lightened and a switching frequency is increased when a load becomes heavy.
In a DC-DC converter using a PFM control technique like the one depicted in FIG. 7, the switching element M1 could be turned on again immediately after it was turned off because the comparator CMP malfunctions due to a noise generated at the timing of the switching element being turned off, which makes output control inaccurate. Therefore, as shown in FIG. 8, a timer TMR2 which defines a minimum OFF period, and AND gate G2 which gains logical conjunction between an output of the timer and an output of the comparator CMP are provided, thus guaranteeing a minimum period for the driving switching element M1 to be turned off in order to prevent the malfunction due to a noise.
However, in the PFM control DC-DC converter driven for a fixed ON period with a guaranteed minimum OFF period, when a load becomes heavy suddenly, a control system operates so that a current IL passing through the inductor L1 is increased as a frequency of driving pulse is increased. Here, the rate of increase in the inductor current IL depends on an on-duty of the driving pulse. The on-duty reaches maximum when the converter operates for the shortest OFF period, i.e., the minimum OFF period, and the inductor current IL increases most rapidly.
When a switching frequency is made higher or an input-output voltage ratio becomes large, a fixed ON period is set short, thereby making the maximum on-duty small. Then, if the maximum on-duty becomes small, a failure may occur where the inductor current IL cannot be increased fast enough when a load gets heavy suddenly as shown in FIG. 9. Moreover, there is a PFM control DC-DC converter which is driven for a fixed OFF period, and such type of converter may have a defect where a decrease in the inductor current IL is not carried out fast enough when a load is lightened suddenly. Incidentally, when a load is lightened suddenly, a converter driven for a fixed ON period is able to respond swiftly to the sudden decrease of the load as it can turn off the switching element M1 continuously even if a sudden load change detection circuit 22 does not operate.
Thus, there is a proposed invention which increases a maximum on-duty cycle by preparing the first and second fixed ON periods and switching therebetween (for example, Japanese Patent Laid-Open Publication 2009-148157). However, in a DC-DC converter having such structure, a plurality of timers are needed, making a circuit scale larger, and an inductor current may not be increased fast enough even in a high-side on-duty cycle.